At higher altitudes a blanket of snow wipes out any sign of a boundary.

Recently the Pakistani army flew foreign journalists along this imaginary line, to demonstrate its determination to seal the border.

The tour was meant to counter Nato and Afghan charges that the Taleban had created a sanctuary in the tribal belt along Pakistan's frontier, giving crucial support to the insurgency in Afghanistan.

Pakistan admits there is some cross-border infiltration, but says it's doing everything it can to stop it.

Spring fears

Much is at stake, because a surge in fighting is expected when spring clears the frozen mountain pathways.

This would be sort of a buffer zone... we could make it very difficult for them [the Taleban]

Maj-Gen Azhar Ali Shah

Our helicopter landed at a notorious insurgent crossing point, near the hamlet of Lwara Mundi in the region of North Waziristan. A red brick army fort sits isolated in an icy plain, facing the gap in the snow-covered mountains.

Maj-Gen Azhar Ali Shah is responsible for North Waziristan, which lies along less than 10% of the 2,400km (1,491-mile) border.

He has 97 checkpoints under his command.

Last year his men stopped a group of returning fighters here at Lwara Mundi.

And this is one of the small sections of the vast and rugged frontier that Pakistan plans to fence.

"We also decided to establish a security zone about 3km-deep in North Waziristan agency," says the general.

"This would be sort of a buffer zone. Although still 100% of those going across may not be checked, we could make it very difficult for them."

And he says, the coalition forces have to do more.

"On the other side certainly there is a need to increase the number of posts, but we understand whenever the troops become available, they would have those posts as well."

Lwara Fort is close to a major insurgent crossing point

The government is eager to show it's on top of border security in North Waziristan, because it has been criticised for ceding control here to pro-Taleban tribesmen.

For more than a year the army waged a military campaign against foreign militants and their supporters in this tribal area. But that backfired. It led to fierce fighting, and radicalised the local tribesmen, turning them towards the Taleban.

So the government signed a peace deal, aimed at returning power to the tribal elders who had lost influence to the pro-Taleban militants.

Around 100 tribal notables gathered at the main army base in North Waziristan, invited by the military to publicly declare their commitment to the agreement in front of the visiting journalists.

Fierce fighting

They sat in chairs set up on a neatly trimmed green lawn, black or golden turbans piled high on their heads.

"We stand by the peace deal, no one can break it," said their designated spokesman, Gul Abad Khan.

"There are problems, but that's because of foreigners, the local tribesmen are bound by the agreement, and we are not facing any kind of local Taleban resistance."

Still, observers aren't convinced the elders have the will, or the authority, to act.

The peace deal has reduced fierce fighting between the army and the locals, but it hasn't stopped cross-border infiltration, and it's widely reported to have empowered the pro-Taleban tribesmen on the ground.

It certainly looks that way in neighbouring South Waziristan, where the government has also signed a peace deal with the tribes.

There, militants recently took journalists on a tour, openly carrying weapons and driving confidently, apparently without fear of challenge.

"We will continue to fight against the infidel foreign troops until they are thrown out," their leader, Baitullah Mehsud, told a BBC reporter, admitting that he sends his men to fight in Afghanistan.

"God willing at the end they will be defeated. We will force them to leave Islamic countries."

The Pakistanis say they will continue to hunt militants like this, but only alongside a political process.

They say force alone won't work on either side of the border. It was the military campaign which turned the tribes against the army in Pakistan, and the same thing is happing in Afghanistan, says Ali Jan Mohammed Aurakzai, governor of North-West Frontier Province.

"Today they've reached a stage that a lot of local population have started supporting the militant operations," he told journalists.

"And it is developing into some sort of a nationalist movement, a resistance movement, a sort of war of liberation against the coalition forces."

The coalition forces strongly reject that argument. They say the Taleban can be defeated by a mixture of military force and economic reconstruction, rather than political accommodation.

The expected military offensive this spring may show more clearly who's right.